Click this link to watch a YouTube Video
about the trackball telescope

An overview of the trackball concept

Kathy and I had been
enjoying amateur astronomy for about a year when I decided it was time
to try my hand at building a telescope. I wasn't happy with equatorial
or Dobsonian mounts--both types have difficulty reaching certain parts
of the sky, and equatorial mounts make you do some back-breaking
contortions to reach the eyepiece once you do find your target--so I
decided to see if I could design something that would be easy to point
and easy to look into no matter where in the sky it was aimed.

I didn't want to be influenced by what others had done, so I
purposefully didn't look for other designs until I had come up with one
on my own. I followed a few false leads, but I eventually figured out
that a spherical base resting in a socket would let me point the scope
anywhere in the sky with equal ease, and it would let me rotate the
eyepiece to a comfortable position no matter where I was looking.

That was half the battle, but one of the things I didn't like about
Dobsonian mounts is that you have to keep shoving them by hand to track
the stars. How could I make my mount track automatically? The answer to
that came in a flash of inspiration: The stars move because the Earth--a
sphere--rotates. But I was building a spherical telescope, so if I made
it rotate in the opposite direction, it would track. Once I realized
that, the solution was obvious: rest the sphere against an axle that
points at the celestial pole, and rotate the axle.

Confident
that I had just reinvented the wheel--almost literally--I went online to
see how other people had done it. Surprise! Nobody had. Spherical
telescopes were old news (Isaac Newton even mounted his scope on a
ball!), but nobody was talking about my kind of mount. I showed a scale model to other astronomers, figuring there
must be some fundamental flaw in the design that would explain why
nobody was building them, but nobody I talked to could find anything
wrong with the concept. And none of them had ever heard of this design,
either.

So I searched the U.S. Patent and Trademark Office database, and
came up blank there, too. As near as I could tell, I had come up with a
new design for a mount that not only eliminates the pointing problems of
equatorial mounts and Dobsonian mounts, but also tracks. I called it the
"trackball" because there's no better name to describe it. Happily,
another search of the USPTO database showed that the term "trackball"
has fallen into the public domain, so apparently anyone is free to use
it as a generic term for this type of telescope.

Many people urged me
to patent the design, but I decided I would much rather put it into the
public domain so anybody could build (and sell) them as they wish. So I
did just that. In July of 2005 I displayed the scope and mount at a
public star party, effectively placing it in the public domain. The
article in Sky and Telescope reinforced that action, as does this
web site. My actions only affect the parts of the design that are new
with me, of course, but that much at least is free for everyone to use,
and nobody may patent it.

After the article came out, I heard from several people who
remembered seeing similar designs in the past. None of those designs
have been quite the same as mine, but the concepts are similar enough to
make it clear that the idea has been around for a while. That's okay
with me! As with any star party, the more the merrier. I've put more
detailed information on the other designs
below.

Speaking of other designs, here's a photo of my second trackball,
which I made for my wife, Kathy. It's nearly identical to the first in
construction and focal length. The biggest difference is that I beefed
up the mount a bit and used a different motor. More on that on the mount page.

So how do you build a trackball? The most important thing is not to
be afraid to experiment. My design philosophy has been to make it work
first, then worry about making it pretty. I'll describe how I built
mine, but if you have a better idea for any of aspect of the design, go
for it! Look around your shop for whatever will work rather than
following my recipe to the letter. So with that in mind, here's a
general idea of how to build a trackball. I've divided the instructions
into two pages to make them easier to load and view. Click on the part
of the top photo you're interested in, or click on the following links.
Below these links I've written some instructions for using the completed
telescope and mount, and some other ideas and random thoughts about the
design.

How to use the telescope

The trackball is amazingly easy to set up. Just put the mount on a
level surface, aim the drive axle at the celestial pole, set the
telescope on the mount, and turn on the motor. This simple alignment is
all you'll need for visual observing even at relatively high power.

To find your target, just push the scope where you want it to go. If
the eyepiece isn't in a comfortable position, rotate the scope until it
is. Most times you can sit down while you're observing, which is by far
the most comfortable way to view.

Once you've found your target, just let go and the scope will track.
You can center it east-west (right ascension) with the fast or slow
tracking buttons, but north-south (declination) adjustments are made by
nudging the scope by hand.* Also, if there's any play in the gears (and
there probably will be), then you'll want to overshoot to the west and
then pull the scope back to the east to load the gears so the scope will
begin tracking the moment you let go. You'll quickly learn to combine
these adjustments into one smooth, quick motion, so you don't even think
about it anymore. You'll just point, let go, and observe.

If your target drifts out of the field to the east or west, adjust
the tracking speed. If it drifts to the north or south, nudge the mount
sideways or raise or lower the axle a bit to bring it back. That's
called drift aligning, and it's a piece of cake. If you've never done
it, click here for instructions.

*It turns out there's an easy way to make a declination adjustment.
Thanks go to Pierre Lemay (see below) for
the idea. He figured out that you can make a declination adjustment by
mounting the idler bearings on a pivot that lets you move them toward
and away from the drive axle. That will force the ball to roll up or
down the axle, which translates to motion in declination. Very neat!
I've got a couple other thoughts about this concept on the mount page.

A few other random thoughts

Cooling

There's not a lot of air flow inside the ball. My scope takes about
half an hour to cool on an average night, but then again, my mirror is
pretty thin. If yours is thicker, it might take longer, and you might
need to install a fan. I've experimented with a 1.5" muffin fan mounted
at an angle inside the ball where it's out of the light path and can
swirl air down under the mirror and back out the other side of the ball,
and that seems to work okay. You might be able to drill holes in the
ball and mount a fan to blow air straight out (or in), but if you do,
use a small drill so the holes don't mess up your tracking. They have to
be small enough that the grid of them will ride across the bearings
without bumping.

You might try putting insulation over the counterweight to keep it
from radiating heat into the ball, but that might just slow down the
cooling process. It depends on how fast its heat will escape through the
outer surface of the ball. Experiment with removable foam before you
spray permanent stuff in there!

Nesting

You could cut a big enough hole in the ball for the secondary cage
to nest in there during transport, but that would probably cut into your
ability to look close to the horizon. I thought collapsibility would be
a neat thing when I was designing my scope, but I quickly gave it up
when I saw how difficult it would be, and in truth I've never needed to
take the telescope apart except to tinker with it anyway. Maybe if it
was longer, but at 38 inches it fits into the back seat (or upright in
the passenger seat) of my Volkswagen just fine.

Carrying

I haven't put a handle on mine yet, but the part of the ball
directly below the eyepiece never rides on the bearings, so a handle
there shouldn't get in the way of anything. On the other hand, it's
pretty easy to carry the scope by the lip of the ball, so I may just
keep doing that. One other thought on the subject: I'm not sure how
strong an acrylic sphere is. My fiberglass sphere is plenty strong
enough to support the entire scope's weight from a couple of handle
attachment points, but you might want to err on the side of caution with
an acrylic sphere.

Ideas to experiment with

(Any of these that are not already patented I also place in the
public domain)

The ball could be made of an open grid of wires or hoops or
whatever. You would need larger bearing surfaces--maybe slides--so the
gaps wouldn't affect the tracking, but it would really help with
cooling (and maybe with weight, too).

The axle could be a conveyor belt or a sling or anything else
that pushes the ball. It could be spring-loaded to press into the ball
while tracking and moved away while slewing. (This would allow the use
of a very grippy substance for the axle, which could help eliminate
fussiness over balance.)

The mount could be any kind of a cradle, even a toilet-seat-style
hole, with the axle pressed against the scope (maybe spring loaded as
mentioned above).

Rotation could be constrained to the correct direction with
multiple bearing surfaces oriented in the proper direction, or with a
suction cup on a bearing that stuck to the bottom of the ball. (That
would probably have to be on a lever so it could be moved away while
slewing.)

The axle could be cone-shaped, so variable speed could be
accomplished by moving the axle longitudinally rather than changing the
motor speed.

You could paint a star map on the ball and use a pointer on the
mount to help you aim the scope. Of course when you rotate the scope for
a comfortable eyepiece position, your map will move...

You could use the telescope itself for precise latitude
alignment. Drift align the scope once, and after you've got it tracking
perfectly, put Polaris (or Sigma Octantis if you're in the southern
hemisphere) in the center of the view of a medium-power eyepiece. Mark
the point where the ball and the axle come together, then next time
you set up just sight on Polaris (or Sigma Octantis) and move the axle
up and down until the marks line up. You could probably use a similar
method for left-right alignment using the idler bearings.

You could use magnets for fine-tuning the counterweight system.
Overdo the weight inside the ball by a pound or so, and put a metal
strip up near the focuser. Stick magnets on the metal strip, and
remove them when you put a heavy eyepiece in. Or you could embed a
metal strip in the ball (easiest if you're making a fiberglass ball)
on the side opposite the focuser and stick a few magnets to that when
you use a heavy eyepiece.

Credit for this idea goes to Pierre Lemay (see
below). You can make a declination adjustment by mounting the idler
bearings on a pivot that lets you move them toward and away from the
drive axle. That will force the ball to roll up or down the axle, which
translates to motion in declination. Very neat! You could get the same
effect by moving the drive axle in and out (being careful not to
change your polar alignment in the process--and being careful not to
push the ball off the other bearings!)

Total cost

I put about $500 into my first trackball. Some of that was blind
alleys (making a fiberglass sphere instead of buying an acrylic one,
experimenting with Teflon instead of bearings, etc). My second and third
ones cost about $350-$400 each. If you buy a finished primary mirror
rather than make your own, it will probably run you more. Or you could
rob the mirrors and other hardware out of your Newtonian and build the
whole thing for practically nothing. Once you use a trackball, you'll
never go back to that old scope anyway. :-)

Here's my
scale model: a Christmas tree ornament with a film can taped to it to
represent the top end of the scope. I smooshed modeling clay inside the
ball to counterbalance the film can, and set it on a triangle of sticks.
When I spun the rubber-hose axle that I placed over one of the sticks,
the film can would trace an arc across the sky. Near the pole, it would
just spin in place. It worked!

The other designs

Several people have emailed me since the article came out in Sky
& Telescope magazine, telling me about other designs for making
sperically mounted scopes track.

The earliest I've heard of so far was built in the early 1970s by
Norman James. His scope floated in a pan of water and was driven with an
axle attached to the low end of the ball by a suction cup. Here are
links to a couple of pictures of that:

Then there's a Belgian man named Alphonse Pouplier who motorized an
Astroscan and had an article about it in the August 1993 Sky &
Telescope. He didn't use a polar-aligned axle, but he did come up
with a clever tracking system--and his even has go-to capability. He has
posted his article online; the URL is http://users.skynet.be/alphonse/skytel.htm

The third, and so far closest design to mine, was built by Pierre
Lemay of Quebec. He tried a single drive axle with two omnidirectional
idler bearings and had trouble with the axle slipping against the
sphere, so he switched to two drive axles and a single idler. That gave
him better control of the ball, and also allowed him to devise a very
clever way to make fine adjustments in declination. He mounted his idler
bearing on a pivot so it can move toward or away from the drive axles.
That forces the ball to roll up or down the axles, which provides
declination control.

Pierre also arrived at the same conclusion I did regarding patents.
He decided to put his idea in the public domain, so he showed it at the
1995 Stellafane convention, and it was featured in the "Ten Top
Telescope Ideas of 1995" article in the January 1996 issue of Sky
& Telescope.

Pierre would be
happy to discuss his design with anyone who is interested. His email
address is to the right. (Sorry you can't click
on it or copy and paste it; it's a graphic file to thwart spambots that
search the internet for addresses to send junk mail to.)

Drift alignment

Drift alignment can seem pretty involved if you get a complicated
set of instructions, but it's actually very simple.

In the northern hemisphere, do this:

Aim the telescope at a star to the south, and somewhere near the
celestial equator. (That's 90 degrees away from the pole, along a line
that runs straight overhead.) Look for north-south motion of the star.
We're only concerned with north-south motion. If it drifts east or west,
your drive is set too fast or slow. And I'm talking about actual motion,
i.e. the direction you have to push the telescope to re-center the star.
If the star drifts south, then rotate the entire mount so the axle
points farther west. If it drifts north, rotate the mount so the axle
points farther east.

Now look at a star near the horizon to the east, and see if it
drifts north or south. This time if your target drifts south, then angle
your axle farther upward (toward a higher latitude on your scale). If it
drifts north, lower the axle.

If you were a long way off in either direction, do the whole
procedure again to fine-tune it.

In the southern hemisphere, do this:

Aim the telescope at a star to the north, and somewhere near the
celestial equator. (That's 90 degrees away from the pole, along a line
that runs straight overhead.) Look for north-south motion of the star.
We're only concerned with north-south motion. If it drifts east or west,
your drive is set too fast or slow. And I'm talking about actual motion,
i.e. the direction you have to push the telescope to re-center the star.
If the star drifts south, then rotate the entire mount so the axle
points farther east. If it drifts north, rotate the mount so the axle
points farther west.

Now look at a star near the horizon to the east, and see if it
drifts north or south. This time if your target drifts south, then angle
your axle farther downward (toward a lower latitude on your scale). If
it drifts north, raise the axle.

If you were a long way off in either direction, do the whole
procedure again to fine-tune it.

Acknowledgments

I've had a lot of help in developing the trackball idea. My wife,
Kathy, has been incredibly supportive and has helped brainstorm many a
gadget at unlikely hours of the day. David Davis and Mel Bartels, thin
mirror gurus and telescope builders extraordinaire, have also been most
generous with their time and enthusiasm. Thanks also to Ted Touw, Craig
Daniels, Chuck Lott (of the Lott 35mm finder fame), Tom Conlin, Arnie
Wittstein, Jim Ruzicka, Dave Cole, Bill Murray, and the entire Eugene Astronomical Society for
help and support. Thanks also to Gary Seronik, my editor at Sky
& Telescope, who knows how to make a writer feel appreciated.
Thanks to you all, I've had a ton of fun on this project.

After the article came out, I heard from many people who remembered
seeing similar designs, and from one person who had actually built a
similar design over a decade ago. My thanks to all of you who helped
flesh out the history of this idea, and my thanks especially to Pierre
Lemay, who not only arrived at the same design before I did, but who
placed it in the public domain just as I did. Great minds think alike!

I hope anyone else who builds a trackball will get as much enjoyment
from it as I have had with mine.

How to contact me

I'd
love to hear from people who are interested in the trackball. Please
feel free to email me at the address on the right. (Sorry you can't
click on it or copy and paste it; it's a graphic file to thwart spambots
that search the internet for addresses to send junk mail to.) I
have no idea how much mail this idea will generate, so I can't guarantee
a response, but I'll do my best to answer everyone who writes with a
genuine question or comment about the trackball.